chore(doc-gen): include `enum` in the export doc types

Fixes errors about missing "path templates" such as:

```
warn:    No path template provided - doc "ViewType" (enum)  - from file "angular2/src/render/api.ts"
warn:    No output path template provided - doc "ViewType" (enum)  - from file "angular2/src/render/api.ts"
```

docs/dgeni-package/index.js

@@ -29,7 +29,8 @@ module.exports = new Package('angular', [jsdocPackage, nunjucksPackage, linksPac
     'interface',
     'function',
     'var',
-    'const'
+    'const',
+    'enum'
   ];
 })